A. Jos van Dillen

Impact of the structure and reactivity of nickel particles on the catalytic growth of carbon nanofibers

Catalytically grown fishbone carbon nanofibers (CNF), are prepared by the decomposition of carbon-containing gases (CH4, CO/H2 or C2H4/H2) over a silica-supported nickel catalyst and an unsupported nickel catalyst at 550 ◦ C. It turns out that both the nickel particle size and the nature of the carbon-containing gas significantly affects the CNF growth process. We demonstrate that at the chosen temperature small supported nickel particles need a carbon-containing gas with a relatively low reactivity, like CH4 or CO/H2, to produce CNF. The resulting fishbone CNF have a uniform and small diameter (25 nm). The CNF thus synthesized hold great potential, e.g. as catalyst support material. However, the large unsupported nickel particles only produce CNF using a reactive carbon-containing gas, like C2H4/H2. The CNF thus obtained show a variety of morphologies with a large range of diameters (50–500 nm). The CNF yield is a subtle interplay between the nickel particle size and consequently the exposed crystal planes on the one hand and the reactivity of the carbon-containing gas on the other.

Synthesis of supported catalysts by impregnation and drying using aqueous chelated metal complexes

Abstract Work is reviewed on the synthesis of supported metal and metal oxide catalysts using impregnation of an aqueous solution of chelated metal ions followed by drying. The nature of the aqueous solutions of chelated complexes is discussed first. Upon solvent evaporation a steep increase in viscosity is apparent, which inhibits redistribution of impregnated solution upon drying of the support bodies. Furthermore, a gel-like phase is formed that favors high dispersions of the active phase after full drying. Second, several examples are dealt with in some detail, in particular supported iron, nickel, and cobalt–molybdenum catalysts. Finally an overview is presented for metal and metal oxide precursors that can be suitably deposited upon support materials using chelated aqueous metal complex solutions.

Synthesis and characterisation of MCM-41 supported nickel oxide catalysts

Nickel ions were deposited onto hydrothermally synthesised all-silica MCM-41 by means of incipient wetness impregnation with different nickel precursor solutions, viz. nickel nitrate and nickel citrate. The calcined nickel catalysts (10 wt.% Ni) were characterised using nitrogen physisorption, temperature programmed reduction, transmission electron microscopy, X-ray diffraction and X-ray photo-electron spectroscopy. With nickel citrate the highest nickel oxide dispersion was obtained. The nickel oxide particle size was inferred to be very small with this precursor, allowing the particles to be located mainly inside the mesopores of MCM-41. With nickel nitrate a bimodal particle size distribution was obtained and next to small nickel oxide particles inside the mesopores relatively large particles (i.e. >10 nm) were present on the external surface of the support, thus lowering the dispersion of this catalyst. Textural analysis showed that with both preparation methods neither pore-blocking nor structural collapse of the support occurred. From these results it is concluded that incipient wetness impregnation with a nickel citrate precursor solution is an excellent method to prepare MCM-41 supported nickel catalysts combining both a high metal loading and a high dispersion of the active phase.

Preparation, characterization and catalytic testing of cobalt oxide and manganese oxide catalysts supported on zirconia

Abstract Zirconia-supported dehydrogenation catalysts based on manganese oxide and cobalt oxide were investigated. Preparation of zirconia-supported Mn(-K) and Co(-K) catalysts was carried out by (co-) impregnation of zirconia pellets to incipient wetness. Characterization of the fresh catalysts was performed using nitrogen adsorption, electron microscopy (TEM), X-ray diffraction (XRD), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS). The dehydrogenation of 1-butene was used as a catalytic test reaction. It was shown that catalysts containing finely divided cobalt oxide or manganese oxide homogeneously distributed over zirconia supports can be prepared using a pre-shaped zirconia support. From the results obtained with various characterization techniques it was concluded that preparation using complex organic precursors results in the best dispersion of the active phase. The metal-EDTA complex seems the most suitable for producing the desired catalyst systems. Catalysts without potassium carbonate deactivate due to carbon deposition. The deactivation behaviour of supported cobalt oxide is similar to that of supported iron oxide, while supported manganese oxide shows a more gradual deactivation. The Mn- or Co-based catalysts containing potassium carbonate did not show deactivation up to at least 10 h on stream. The activity and selectivity are different, however. The activity ranking was found to be Fe,K > Mn,K > Co,K. The selectivity of the manganese oxide-based system was found to be higher than in the iron oxide-based catalyst.

Cobalt on carbon nanofiber catalysts: auspicious system for study of manganese promotion in Fischer-Tropsch catalysis

STEM-EELS and XPS investigation shows manganese oxide to be closely associated with cobalt nanoparticles supported on carbon nanofibers thereby improving selectivity in Fischer-Tropsch catalysis.

Supported hydrotalcites as highly active solid base catalysts

Mg-Al hydrotalcite platelets with a lateral size of 20 nm were deposited on carbon nanofibers and the resulting supported catalyst exhibited a specific activity in the condensation of acetone four times that of unsupported hydrotalcites due to the higher number of active edge sites.

Highly active cobalt-on-silica catalysts for the fischer-tropsch synthesis obtained via a novel calcination procedure

Using ordered mesoporous SBA-15 as model support and pore volume impregnation of aqueous cobalt nitrate we identified a novel calcination procedure in a diluted NO atmosphere, which enables the preparation of Co 3 O 4 /SiO 2 catalysts that combine highly loadings (15 -18 wt% cobalt) with high dispersions (4-5 nm Co 3 O 4 particles). Experiments with silica gel demonstrated that this method is applicable to conventional supports too. After reduction the Co/SiO 2 catalyst treated via this calcination method showed to have a superior activity in FT synthesis compared to the catalyst treated via air calcination.

Mössbauer spectroscopic investigations of supported iron oxide dehydrogenation catalysts

Abstract The characterization with Mossbauer absorption spectroscopy of supported iron oxide-based catalysts, containing only iron oxide or containing both iron oxide and potassium carbonate, after preparation as well as after exposure to the reaction conditions used in 1-butene dehydrogenation is described. The fresh catalysts all contain well-dispersed iron(III) oxide particles. The potassium carbonate compound remaining on the surface after decomposition of KFeO2 formed during the calcination step disperses the iron oxide phase in the case of the magnesia and zirconia supports. When using titania as a support the formation of a mixed oxide is observed. With the Fe/TiO2 catalyst FeTiO3 has been formed after dehydrogenation. In the Fe/MgO catalyst next to a surface-stabilized Fe1−xO phase, well-dispersed Fe3+ species are present. In the zirconia-supported sample Fe3O4 is detected. When both iron oxide and potassium carbonate are present, the iron-containing phase in the K,Fe/MgO system also consists of a mixture of the Fe1−xO phase and well-dispersed Fe3+ species. In the K,Fe/ZrO2 catalyst besides the Fe3O4 phase a well-dispersed Fe3+ species is observed. However, the magnetite crystallites in this catalyst are notably smaller than in the Fe/ZrO2 catalyst after dehydrogenation. No distinction between a well-dispersed (α)Fe2O3 or a well-dispersed KFeO2 phase can be made at this stage. It is most probable that the main functions of the potassium in the supported catalysts can be described as, firstly, dispersing the iron phase, and secondly, providing the gasifying properties required for the auto-regenerative character of the catalyst.